Different dyes are used to impart color to an infinite variety of substrates in both batch and continuous type of processing. Dyes may be either synthetic or natural, water soluble or insoluble. Having a unique chemistry, one dye may be more suitable for a given type of substrate than another. The desired color or shading on a particular substrate will often dictate the dye selected.
For example, disperse dyes such as Disperse Yellow 3 (N-[4-[(2-hydroxy-5-methylphenyl)azo]phenyl]-acetamide) or Disperse Red 55 (1-amino-4-hydroxy-2-(2-hydroxyethoxy)-9,10 anthracenedione) are used extensively on polyester over a full shade range. However, when applied on acrylic fibers, disperse dyes are used primarily for pastel shades. Similarly, vat dyes such as Vat Black 25 (3-(1-anthraquinonylamino)anthra[2,1,9-m,n,a]naphth[2,3-h]acridine-5,10,15 -(16H)-trione) are used almost exclusively for dull color shades on substrates including cotton and rayon. On the other hand, basic (cationic) dyes such as Basic Red 29 (thiazolium) or Basic Blue 41 (Benzothiazolium) have an almost unlimited range of shades with good color value. Basic dyes provide among the brightest colors such as mauve, fuchsia, violet and blue and are employed extensively on acrylics and often on paper, silk and leather. Likewise, azoic dyes such as the Naphthol compounds offer a wide range of shades but are typically used to produce full shades of red, scarlet and burgundy on substrates including cotton, polyester, linen, jute, hemp, and rayon.
Although many dyes can be used in both batch and continuous processing, dye selection may be contingent on the type of dyeing process required. For example, direct dyes such as Direct Black 80 (7-amino-2-{7-[p-(4-amino-6-1-naphthylazo)phenylazo]-8-hydroxy-6-sulfo-2-n aphthylazo}-1-naphthol-3-sulfonic acid) used on a variety of substrates including cotton and rayon are processed continuously or in batch. The same is true for azoic dyes and sulfur dyes such as Sulfur Black 1 (constitution unknown). However, the acid dyes such as Acid Blue 40 (2-Anthracenesulfonic acid) or Acid Orange 156 (Benezenesulfonic acid) originally were devised exclusively for dyeing wool and will typically undergo batch processing in order to acquire uniformity of color.
Regardless of the physical and chemical nature of a dye or substrate employed, it is always important to have the correct solution pH. A good general description of the coloration and dyeing processes for many of the above listed substrates can be found in DYEING PRIMER, a series of short papers on the Fundamentals of Dyeing in Textile Chemist and Colorist. The following articles from Textile Chemist and Colorist are included: "What are Dyes? What is Dyeing?" by J. Richard Aspland, Vol. 12, No. 1, 1980; "Dyeing With Acid Dyes" by J. Lee Rush, Vol. 12, No. 2, 1980; "Dyeing With Basic Dyes" by Mathias J. Schuler, Vol. 12, No. 3, 1980; "Dyeing with Direct Dyes" by Marshall White, Jr., Vol. 12, No. 4, 1980; "Dyeing With Vat Dyes" by Claude S. Hughey, Vol. 12, No. 5, 1980; "Dyeing With Sulfur Dyes" by Leon Tigler, Vol. 12, No. 6, 1980; "Dyeing With Azoic Dyes" by Herbert B. Moore, Jr., Vol. 12, No. 7, 1980; "Dyeing With Disperse Dyes" by Mathias J. Schuler, Vol. 12, No. 8, 1980; "Dyeing With Reactive Dyes" by Peter J. Dolby, Vol. 12, No. 9, 1980; "Special Coloration Techniques" by J. Richard Aspland, Vol. 12, No. 10, 1980; "The Application of Color Technology in Today's Textile Industry" by Ralph Besnoy, Vol. 12, No. 11, 1980; "Kinetics and Equilibria in Dyeing" by Ralph McGregor, Vol. 12, No. 12, 1980.
A dyeing solution must maintain the proper pH to provide accurate and consistent shading of color. This applies to virtually any type of dye or substrate regardless of the mechanical processing employed. Control of pH in a dyeing process is critical and is a function of many factors including: the dye, the amount of dye used, the chemistry of the application medium (typically water), the rate of temperature change of the dyeing process, and the rate and method of dye exhaustion onto the substrate.
In order to preserve proper pH, chemical buffering systems are incorporated into dyeing solutions. A chemical buffering system is one that maintains the correct acidity or alkalinity of the dyeing solution and consists of a weak acid or weak base and its salt. The combination or concentrations of the weak acid/base and its salt determines the buffering range and capacity. Commonly used prior art systems for buffering a dyeing solution and controlling pH include ammonium sulfate, phosphoric acid and acetic acid. The availability of these chemicals and their ability to lower the dye bath pH has made them desirable.
Ammonium sulfate, for example, is a very common pH control for a variety of dyes and substrates. Azoic dyes, disperse dyes, vat dyes, acid dyes, and basic dyes have all utilized ammonium sulfate to control pH. A conventional prior art process using ammonium sulfate pH control consists of a solution made up of about 2% ammonium sulfate having water as the application medium. 1-2% leveling agent and 0.25% surfactant are then added. The substrate such as nylon or polyester is introduced into the bath at about 40.degree. C. and runs without the dye for 5 minutes. Once the dye is added, the solution is heated by introducing stem for 25-35 minutes to complete the dyeing process. Here, the ammonium sulfate is used to control pH and maintain an acidic dye solution. Steam is employed at either atmospheric pressure or under pressure to each and maintain a near boiling temperature.
Another prior art process which uses phosphoric acid as the pH control employs a solution of about 0.50% phosphate buffer (which includes the phosphoric acid) and 0.50% surfactant. The dye is added to cold water in a batch process and then steamed until well mixed with water. All other chemicals such as antifoams and water softeners are added except the phosphate buffer. The batch is circulated for 3-5 minutes. Thereafter, the substrate is added and circulated for 2 minutes. The buffer is then added and the temperature is raised to 180.degree. F. at a rate of 4.degree. F. per minute using steam. Once the 18020 C. temperature is maintained for 5 minutes, a substrate sample is tested for accuracy and consistency of shading.
Many such prior art methods of maintaining proper pH in dyeing solutions are considered hazardous and toxic according to current environmental regulations. For example, acetic acid, ammonium sulfate and phosphoric acid all enhance microbial growth in receiving water systems such as lakes and rivers. These microorganisms require nutrients encompassing a variety of carbon compounds such as acetate from spent acetic acid, nitrogen from ammonium sulfates, and phosphates from phosphoric acid. The bacteria also consume large amounts of oxygen indicative of an increase in the Biological Oxygen Demand (BOD) of the water.
Consequently, the bi-products of the prior art methods if discharged into a water system will escalate the growth of bacterial. Hence, the oxygen level is depleted leaving little if any oxygen for aquatic growth such as fish. The result is a lifeless water stream and an imbalance in the ecosystem. Therefore, prior art methods of pH control require careful effluent treatment and disposal.
In addition, a bi-product of an ammonium sulfate buffer is an ionized form of ammonia that cannot be leached into an effluent water going into city waste treatment system. Further, the acetic acid method of pH control results in zinc removal from the latex backing of conventional textile materials such as "scatter" rugs. It is very difficult to dispose this material.
As a result, the dye industry has been seeking new methods to maintain and control pH that obviate the use of such chemicals as acetic acid, ammonium sulfate and the like. Moreover, environmental problems with effluent discharge have caused dyers to incorporate more exacting controls in the dyeing operations while looking for new methods to monitor pH. Many prior art methods only add the pH adjusting chemical(s) such as acetic acid, during the initial batch formulation and do not provide an ongoing capability to adjust the pH during the dyeing cycle. Without capability to continuously adjust the pH, rework is frequently necessary and consequently more dye is utilized. Therefore, better methods of repeatability which lessen the amount of rework have been sought.